This invention relates to an electrode substrate, in which wires reduce resistance of electrodes for applying voltage to a pixel so as to respond to a large-capacity and high-definition display, and a manufacturing method thereof, and further concerns a liquid crystal display element which is provided with the electrode substrate.
Ferroelectric liquid crystal display has superior properties in memory, fast response, and a wide viewing angle. In addition, as for a liquid crystal display device using ferroelectric liquid crystal, when direct matrix method is adopted, in which stripe scanning electrodes and signal electrodes made of transparent conductive films are arranged in a matrix form on a substrate, it is possible to provide a display with a large capacity and high definition as compared with a TN(Twisted Nematic) and an SNT(Super-Twisted Nematic) liquid crystal display devices that also adopt a direct matrix method. This advantage of ferroelectric liquid crystal is described in xe2x80x9cApplied Physics Letters 36, (1986) p.899-901xe2x80x9d by N. A. Clark and S. T. Lagerwall.
However, as for ferroelectric liquid crystal adopted for a direct matrix method, when merely a transparent conductive film is used for forming the stripe electrodes so as to manufacture a ferroelectric liquid crystal display device having a large capacity and high definition, it is necessary to form long stripe electrodes in accordance with a wider display area, resulting in larger resistance on electrodes. Consequently, problems such as heat, delay of a signal, and a deformed waveform of a signal applied to a pixel area, tend to occur and affect a drive.
The conventional TN and STN liquid crystal display devices form a high contrast screen by adopting a plurality of frame scans which use a multiplexing drive for periodically applying driving voltage. Therefore, delay effect, which causes a delay in applying driving voltage, has hardly degraded display quality. However, recently, as for such a liquid crystal display device, in order to respond to the growing needs for a larger screen and faster response, influence of the delay effect cannot be ignored.
For this reason, when a larger screen is provided in the ferroelectric liquid crystal display device, a method has been adopted, in which low-resistance conductive wires such as a metal film are provided for reducing the entire electrode resistance.
The conductive wires are formed so as to conductively come into contact with the stripe electrodes along the length of the stripe electrodes. Meanwhile, as for the ferroelectric liquid crystal cell and the STN liquid crystal cell that need to be manufactured with a small cell gap, evenness on a substrate surface significantly affects liquid crystal alignment. Therefore, as for such a liquid crystal cell, it is necessary to obtain favorable evenness on the substrate. Hence, the conductive wires need to be arranged so as to realize favorable evenness on a substrate.
In order to form such conductive wires, the following four methods have been conventionally adopted:
A first method adopts a abrading operation as described in xe2x80x9cNEPCON West ""89, p426-p447, 1989xe2x80x9d. This method forms conductive wires in accordance with the steps of FIGS. 13(a) through 13(c). Firstly, metal wires 103(conductive wires) are formed in stripes on a substrate 101, which is coated with polyimide 102 (FIG. 13(a)). Next, on the substrate 101, an insulating film 104 is formed so as to cover the metal wires 103(FIG. 13(b)). And then, merely bumps of the insulating film 104, that are located on the metal wires 103, are abraded so as to expose the upper surface of the metal wires 103(FIG. 13(c)). Further, a transparent conductive film is formed in stripes on the metal wires 103.
This method makes it possible to increase the thickness of the metal wires 103 as well as to expose the upper surfaces of the metal wires 103 merely by abrading the insulating film 104.
A second method adopts photolithography as described in xe2x80x9cIEEE/CHMT ""89, Japan IEMT Symposium, p128-131xe2x80x9d. This method forms conductive wires in accordance with the steps of FIGS. 14(a) through 14(d). The metal wires 103 are formed into stripes on the substrate 101(FIG. 14(a)). Next, negative photoresist(polyimide negative photoresist) is made into a film so as to cover the metal wires 103 on the substrate 101; thus, an insulating film 105 is formed (FIG. 14(b)). And then, the insulating film 105 is partly removed on the metal wires 103 by photolithography which uses a photo mask 106 for exposing the upper surfaces of the metal wires 103 (FIG. 14(c)).
As shown in FIGS. 15(a) and 15(b), the photolithography process adopts the photomask 106 which includes a plurality of small holes 106b on both sides of a stripe pattern 106a. This arrangement makes it possible to remove projecting portions 105a of the insulating film 105, that are located on both sides of the metal wires 103. Consequently, the surface of the insulating film 105 is evenly formed together with the upper surface of the metal wires 103(FIG. 14(d)). The small holes 106b make it possible to adjust the exposure amount so as to soften and remove merely the projecting portions 105a around the metal wires 103.
This method makes it possible to form the thick metal wires 103 in the same manner as the first method. In addition, photolithography exposes the upper surfaces of the metal wires 103.
As disclosed in Japanese Published Unexamined Patent Application No. 76134/1996 (Tokukaihei 8-76134, published on Mar. 22, 1996), a third method forms stripe conductive wires on a transparent substrate and fills UV(ultraviolet)cure resin between the conductive wires. This method forms conductive wires in accordance with the steps of FIGS. 16(a) through 16(d).
Firstly, a smoothing mold 108, which has UV cure resin 107 applied thereon, is disposed so as to oppose the transparent substrate 101 on which the metal wires 103 are formed into stripes(FIG. 16(a)). Next, the UV cure resin 107 is exposed to ultraviolet light from the back of the substrate 101 so as to form an insulating film having the same thickness as the metal wires 103 (FIG. 16(b)). And then, the smoothing mold 108 is separated from the substrate 101(FIG. 16(c)), and transparent electrodes 109 are formed on the surface of the layer consisting of the metal wires 103 and the UV cure resin 107(FIG. 16(d)).
In this method, in the step of FIG. 16(b), the smoothing mold 108 having the UV cure resin 107 applied thereon is pressed onto the substrate 101 having the metal wires 103 formed thereon, and then, the UV cure resin 107 is exposed to ultraviolet light; therefore, it is possible to achieve a preferable evenness of the insulating film including UV cure resin 107.
As described in xe2x80x9cJ. Electrochem. Soc.; SOLID-STATE SCIENCE TECHNOLOGY August 1988, p2013-p2016xe2x80x9d, a fourth method adopts a liquid-phase deposition film forming method(Liquid-phase deposition; LPD) of SiO2 amorphous film. The LPD method uses solution of hydrosilicofluoric acid(H2SiF6:HF), and the chemical equilibrium of the solution is shifted to the deposition side of SiO2 so as to form a film. This method forms conductive wires in accordance with the steps of FIGS. 17(a) through 17(d).
Firstly, a metallic material is formed into a film on the substrate 101(FIG. 17(a)), and the metallic material is patterned by using photoresists 110 so as to form the metal wires 103(FIG. 17(b)). Here, after the patterning operation, the photoresists 110 are not removed. And then, SiO2 films 111 are formed between the metal wires 103 by using the LPD method(FIG. 17(c)), the photoresists 110 are exfoliated from the metal wires 103(FIG. 17(d)). This method makes it possible to provide an even construction which has no projection or groove on the metal wires 103 and the SiO2 films 111.
However, it is difficult to put the above-mentioned methods into practical use due to the following disadvant ages:
The first method cannot evenly abrade the insulating film 104. For example, portions between the metal wires 103 tend to be abraded as compared with portions disposed on the metal wires 103. Namely, the first method exposes the upper surfaces of the metal wires 103 merely by abrading. However, in order to abrade merely the insulating film 104 on the metal wires 103, an abrading member having superior evenness needs to be contact merely with the insulating film 104 on the metal wires 103. Additionally, if abrasive is used as an abrading member, the insulating film 104 is abraded between the metal wires 103 while the insulating film 104 is abraded on the metal wires 103, resulting in a concave surface between the metal wires 103. Therefore, it is not possible to uniformly abrade the insulating film 104 so as to achieve an even surface.
In the second method, the metal wires 103 formed in stripes, a polyimide negative photosensitive resin is formed into a film as the insulating film 105, and photolithography using the photomask 106 exposes the upper surfaces of the metal wires 103. In this case, the small holes 106b are formed on the photomask 106 so as to remove the projecting portions 105a of the insulating film 105 around the metal wires 103. However, in the second method, the small holes 106b control the exposure amount so as to adjust a film thickness of the hardening insulating film 105; thus, it is always necessary to stabilize the exposure amount. Particularly, this methods uses a negative photosensitive resin, so that the exposure amount must not be excessive. In addition, as disclosed in the aforementioned xe2x80x9cIEEE/CHMT ""89, Japan IEMT Symposiumxe2x80x9d, the insulating film 105 has a thickness of 25 xcexcm and the projecting portions 105a have height h of nearly 3 xcexcm. This method is not a method for forming electrodes of a ferroelectric liquid crystal cell; therefore, the insulating film 105 has a film thickness larger than an insulating film of the ferroelectric liquid crystal cell, so that the projecting portions 105a are higher. Hence, it is understood that the second method has accuracy of merely about 3 xcexcm for removing the projecting portions 105a. 
As for the third method, in order to prevent air bubbles from entering the UV cure resin 107, it is necessary to press the substrate 101 onto the smoothing mold 108 under a reduce pressure(vaccum chamber) in a decompression chamber. Further, a drive is required for pressing the smoothing mold 108 onto the substrate 101. Additionally, this method requires the smoothing mold 108 to be washed for each use. Thus, this method has a drawback of such a complex manufacturing process.
As for the fourth method, the metal wires 103 need to have chemical resistance against hydrofluoric acid; thus, materials for forming the metal wires 103 are limited. Further, in LPD method, the film forming speed, 300 xc3x85/h, is extremely low, so that forming a 1 xcexcm thickness film demands 30 hours. Moreover, in this method, a chemical reaction forms a film, so that the film forming speed is affected by concentration of each ingredient of the silicofluoride solution. Therefore, upon forming the SiO2 film 111, it is necessary to strictly control the concentration of the hydrosilicofluoric acid solution.
The present invention is devised in order to overcome the above-mentioned problems. The objective is to manufacture an electrode substrate in which dielectric wires are formed so as to conductively come into contact with transparent electrodes on an insulating film, in a practical manner, and to provide an electrode substrate which has superior evenness.
In order to achieve the above objective, the electrode substrate of the present invention, which includes a plurality of conductive wires formed on the substrate, a resin film formed between the conductive wires, and an electrode film formed on the conductive wires so as to be conductively contact with the conductive wires, is characterized in that a height difference is 0.11 xcexcm or less between (a) projecting portions formed on the resin film around the ends of the conductive wires and (b) the virtually even surface of the resin film at a portion where the conductive wire is not formed; and a height difference is 0.11 xcexcm or less between the projecting portion and the surface of the conductive wire.
In the above arrangement, it is desirable that no projecting portion appear around the side ends of the conductive wire, regardless whether a thickness of the resin film is larger or smaller than that of the conductive wire at a portion where the conductive wire is not formed. However, when a height difference is 0.11 xcexcm or less, more preferably 0.1 xcexcm or less, or particularly preferable 0.05 xcexcm or less between the virtually even surface and the projecting portion of the resin film, it is possible to obtain sufficiently high evenness on the resin film. Further, when a height difference is 0.11 xcexcm or less, preferably 0.1 xcexcm or less, or particularly preferable 0.05 xcexcm or less between the projecting portion and the surface of the conductive wire, it is possible to obtain sufficiently high evenness between the resin film and the conductive wire. Therefore, when the electrode substrate of the present invention is, for example, adopted for a liquid crystal display element, it is possible to obtain high evenness for the films formed on the electrode film; thus, the liquid crystal alignment and the switching property can be favorably maintained. Consequently, it is possible to achieve a property which provides high-contrast and even display.
As a result of diligent consideration given by the inventor of the present application, if the metal wires 103 are formed in the ferroelectric liquid crystal cell by using the above-mentioned second method, the height h of the projecting portions 105a, that are disposed around the metal wires 103, depends upon a thickness of the insulating film 105(equals to a thickness of the metal wires 103) and is around 0.5 xcexcm at the maximum. Further, a thickness of the metal wire 103, that is found by a resistance value, is about 1 to 3 xcexcm. In order to allow the metal wires 103 and the insulating film 105 to form an even surface, it is necessary to work the insulating film 105(insulating film 105 around the metal wires 103), which has a thickness of 1.5 to 3.5 xcexcm including the projecting portions 105a having the height h of 0.5 xcexcm, so as to remove the projecting portions 105a and have the film thickness of 1 to 3 xcexcm.
However, as described above, the second method has accuracy of nearly 3 xcexcm for removing the projecting portions 105a; thus, it is understood that it is difficult to cure the insulating film 105 with a thickness of 1 to 3 xcexcm and to remove merely the projecting portions 105a having the height h of 0.5 xcexcm while curing the insulating film 105 with a thickness of 1 to 3 xcexcm. Therefore, even when the photomask 106 having the small holes 106b are used, there is a possibility that the ferroelectric liquid crystal element fails to achieve desired evenness on the substrate. Meanwhile, the present invention makes it possible to provide an electrode substrate, which achieves high evenness and obtains a liquid crystal element having the above-mentioned property, in a positive and stable manner.
In order to achieve the above objective, the manufacturing method of the electrode substrate of the present invention is characterized by including the following steps:
(1) a first step for forming a plurality of conductive wires on the substrate,
(2) a second step for forming the resin film so as to cover the conductive wires and portions between the conductive wires on the substrate where the conductive wires are formed,
(3) a third step for removing portions of the resin film that cover the conductive wires so as to expose at least a part of the surface of the conductive wire,
(4) a fourth step for partially removing projecting portions which appear on the resin layer around the ends of the conductive wires due to the exposure of the conductive wires so as to set a height difference at 0.11 xcexcm or less between the projecting portion and the virtually even surface of the resin film at a portion where the conductive wire is not formed, and to set a height difference at 0.11 xcexcm or less between the projecting portion and the surface of the conductive wire, and
(5) a fifth step for forming an electrode film which is formed on the resin film and the conductive wires so as to be conductively contact with the conductive wires.
With the above-mentioned method, upon partially exposing the surface of the conductive wires in the third step, the projecting portions, which appear on the resin layer around the ends of the conductive wires due to the exposure of the conductive wires, are partially removed so as to set a height difference at 0.11 xcexcm or less between the projecting portion and the virtually even surface of the resin film where the conductive wires are not formed, and to set a height difference at 0.11 xcexcm or less between the projecting portion and the surface of the conductive wire; thus, high evenness can be achieved on the surface of the resin film and between the surface of the resin film and the surface of the conductive film. Hence, in the fifth step, it is possible to form the electrode film in a virtually even manner on the resin film and the conductive wires.
Therefore, when the electrode substrate manufactured in the above steps is adopted for the liquid crystal display element, it is possible to evenly form the films provided afterwards on the electrode film. Hence, the liquid crystal alignment and the switching property can be maintained in a favorable manner.
Therefore, without a conventional manufacturing method for completely removing the projecting portions of the resin film, the manufacturing method of the electrode substrate of the present invention makes it possible to readily manufacture the liquid crystal display element which is superior in evenness, at low cost in a practical manner.
Furthermore, the above method is more practical and easy because this method forms the resin layer with a thickness smaller or larger than that of the conductive wire, at portions where the conductive wires are not formed.
In order to achieve the above objective, the liquid crystal display element of the present invention, in which liquid crystal is sandwiched between a pair of the electrode substrates opposing each other, is characterized in that at least one of the electrode substrates is the electrode substrate of the present invention.
As described above, the liquid crystal display element adopts an electrode substrate achieving high evenness, so that the alignment and the switching property of the liquid crystal can be favorably maintained.
In order to achieve the above objective, the liquid crystal display element of the present invention, in which liquid crystal is sandwiched between a pair of the electrode substrates opposing each other, is characterized in that at least one of the electrode substrates is formed by using the manufacturing method of the present invention.
As described above, the liquid crystal display element also adopts an electrode substrate achieving high evenness, so that the alignment and the switching property of the liquid crystal can be favorably maintained.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.